Although synthetic fiber materials possess a variety of excellent physical and chemical properties, a common drawback is their ready tendency to combustion, for which reason such synthetic fiber materials have traditionally been subjected to finishing treatment for flame retardancy. Examples of finishing treatment methods include dip coating and spray coating. An obvious disadvantage of coatings is that they can wear off over time.
Unfortunately, the use of flame retardant additives in synthetic fiber materials can result in undesirable color development or blooming. In other words, such additives can detrimentally affect the color of the synthetic fiber. In many applications, this color development problem is undesirable or unacceptable.
Furthermore, when blended with synthetic fiber materials, flame retardant additives tend to form nonuniform distributions in the mixture, and thus result in nonhomogeneous flame-retarded fibers, which have poor mechanical properties. The problems associated with nonhomogeneity are more pronounced for finer fibers. Specifically, if the flame retardant additive is not completely mixed with the synthetic fiber material prior to extrusion, the flame retardant additive tends to agglomerate onto itself, producing lumps in the extrusion melt, clogging the spinneret filters or orifices, or producing weak spots in the resulting continuous filament. Such agglomerated lumps also necessarily produce other areas in which there will be a lack of flame retardant additive, and thus reduced flame retardant effectiveness.
In particular, brominated polystyrenes have oftentimes been deficient in their color characteristics. Manufacturers of products containing flame retardants generally find it advantageous to have available a flame retardant which will not contribute excessive color to the products or otherwise interfere with color matching specifications applicable to a given product. Thus in general, the lower the color of the brominated styrenic polymer (i.e., the whiter the flame retardant), the better. However, high-melting flame retardants that are white often end up acting as a white pigment in the fiber, making coloring the fibers more difficult, as the white color of the flame retardant must be overcome.
Additionally, relatively few flame retardants work well with polyamides (nylons), at least in part because polyamides are high-melting. Upon oven aging for one week at 180° C., blends of polyamide with a flame retardant tend to develop color.
It would be of considerable advantage if a way of forming more uniform mixtures of flame retardants and fiber-forming thermoplastic polymers could be found, such that more homogeneous fibers may be formed. It would be additionally advantageous if the flame retardants used in such mixtures exhibited minimal color development, and at the same time did not act as a white pigment.